Subject: Applied Physics 2

Topic: Motion Of Charged Particles In Electric And Magnetic Fields

Difficulty: Medium

CRT resembles to horizontally placed conical flask sealed at one end. It consists of electron gun assembly, deflection system and fluorescent screen.

1. electron gun assembly It consist of indirectly heated cathode. Cathode is a nickel cylinder and it encloses filament F. On the passage of electric current through the filament electrons are thermionically emitted. The cathode is surrounded by control grid G held at negative potential. It acts as a gate and regulate the passage of electron. More is The negative potential on the grid less is the electron passed. Grid is brought to Intensity control on the front panel of CRO.$A_1$ ,$A_2$ and $A_3$ are preaccelerating, focusing and accelerating anodes. $A_1$ and $A_3$ are maintained at relatively more positive potential than $A_2$ ..As a result electrostatic lens is formed between electron gun assembly and $A_2$ and $A_2$ and $A_3$ which focus and defocuses electron beam.

2. Deflection systems:

On its way towards the screen electron beam meets two sets of deflecting systems. Vertical defection system and Horizontal deflection system. Vertical deflection system consists of vertical deflecting plates mounted horizontally and horizontal deflecting system consist of horizontal deflecting plates mounted vertically, both perpendicular to each other and perpendicular to electron motion. By applying proper potential on the plates electron spot can be moved anywhere on the screen.

1. Fluorescent screen: The inner surface of the circular front face is coated with fluorescent screen usually ZnO which glows when high energy electron strikes it. A conductive coating of graphite called aquadag coating is coated on the flare of the envelope and is internally connected to $A_3$. It completes electrical circuit from screen to cathode. Cathode is emitting electrons in very large number so it has to be replenished. When electron strikes the screen, it tends to become negatively charged and repels electrons that arrive later. So, these electrons are to be carried away. The aquadag coating connected to $A_3$ attracts the secondary electron emitted and returns to cathode via ground